As anthropogenically-mediated biological invasions become more prevalent in marine systems (e.g. with burgeoning global commerce), it is increasingly essential to understand how variation of environmental factors in new habitats will influence the success of these species. The invasive marine macroalga Sargassum horneri was first observed in Long Beach Harbor, CA, USA in 2003 and has since spread throughout the Southern California Bight and along the Baja California Peninsula, MX, where it forms dense, monospecific stands on low intertidal and subtidal rocky reefs. S. horneri can exert negative effects on recipient communities, particularly through spatial preemption of native macroalgae and resulting impacts on primary production and community composition. While some studies have addressed ecological effects of adult S. horneri, no manipulative studies of sensitive early life history stages had been performed to date. This study used (1) laboratory culture and (2) photophysiological experiments to understand how variation in abiotic conditions impacts germling and juvenile S. horneri, and thus facilitates or inhibits its spread. Treatments consisted of an orthogonally-allocated array of temperatures (10, 15, 20, or 25 ̊C), nutrient availabilities (natural or nutrient-enriched seawater), and light levels (50 or 500 μmol photons m 2s-1). Overall, temperature was the strongest driver of S. horneri growth. Moderate temperatures (15 and 20 ̊ C) facilitated the greatest growth and development in juveniles and germlings, while juveniles were able to survive and grow at temperatures (10 and 25 ̊C) that inhibited germlings. However, physiological tolerance of these temperatures required additional resources, such as high light or elevated nutrients, which may not exist in natural habitats within the invasive region. The results of this study suggest that the S. horneri invasion is unlikely to expand latitudinally past known biogeographic breaks at Point Conception and Punta Eugenia under current oceanographic conditions. It is important, however, to consider changing conditions, such as increasing seawater temperature due to global climate change. These findings can aid resource managers in the identification of potentially-vulnerable recipient localities, both within Southern California Bight/Baja California ecosystem and in other regions connected with northeastern Asia through substantial transoceanic trade.